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Calculated and recently measured photoabsorption transition probabilities of the H2 np?1?+ u and np?1? u ? X 1?+ g band systems have been examined with high-resolution (?? = 95-115 m?) electron-impact induced emission spectra obtained previously by Jonin et al. and Liu et al. When localized rovibronic coupling is insignificant, transition probabilities calculated with the adiabatic approximation are found to be generally consistent with experiment. However, in the presence of significant coupling, the transition probabilities obtained from a nonadiabatic calculation of B'1?+ u , D 1?+ u , , D'1?+ u , and 5p?1?+ u state coupling give better agreement with the experimental spectra. Emission yields obtained by comparison of the calculated and experimental spectra are also consistent with the measured predissociation and autoionization yields. In addition, more accurate excitation and emission cross sections and nonradiative yields have been obtained for a number of the np?1?+ u and np?1? u states. The results obtained in the present investigation lead to a significantly more accurate calibration of the Cassini UVIS instrument and laboratory spectrometers in the range 790-920 ?. They are also an important step toward an accurate model of extreme ultraviolet H2 auroral and dayglow emissions in the outer planet atmospheres.

Rotationally resolved molecular hydrogen transitions originating from excitation of highly excited rovibrational levels of the X 1Σ state have been systematically identified for the first time in the Far Ultraviolet Spectroscopic Explorer (FUSE) observation of comets C/2000 WM1 (LINEAR) and C/2001 A2 (LINEAR). Spectral assignments for the observed lines of H2 and other atomic and molecular species are given. All observed H2 transitions belong to the Lyman (B 1Σ-X 1Σ) and Werner (C 1Πu-X 1Σ) band systems. Solar Lyα fluorescence excitation of highly rovibrationally excited H2 X 1Σ is found to be almost solely responsible for the observed H 2 emission. Resonant excitation of H2 by Lyβ and other solar lines is very limited.

Medium-resolution (full width at half-maximum Δλ = 0.5 A) vacuum ultraviolet (VUV) emission spectra of H2 from forbidden excitation to the excited singlet-gerade states and subsequent cascade to the singlet-ungerade states have been studied with a time-resolved electron-impact technique. Emissions from the B 1Σ, C 1Πu, B' 1Σ, and D 1Πu states that are entirely due to cascade from the singlet-gerade states have been observed. Lyman band emission intensities dominate over the entire 940-1650 A wavelength region, though Werner band emission intensities are also significant. In addition, very weak cascade emissions from the vj = 0 and 1 levels of the B' 1Σ and D 1Πu states are also evident. In contrast to previous assumption, the cascade excitation is observed to take place over all the vibrational levels of the B 1Σ and C 1Πu states, while contributing heavily to low vj levels of the B 1Σ state. Synthetic spectra based on the calculated transition probabilities of the EF 1Σ, GK 1Σ, H 1Σ, I 1Πg, J 1Δg, O 1Σ and P 1ΣB 1Σ, C 1Πu, B' 1Σ, and D 1Πu band systems and Franck-Condon factors of the EF 1Σ-X 1Σ, GK 1Σ-X 1Σ, H 1Σ-X 1Σ, I 1Πg-X 1Σ, J 1Δg-X 1Σ, O 1Σ-X 1Σ, and P 1Σ-X 1Σ are in good agreement with the observed Lyman and Werner band intensities. The present study shows that a number of singlet-gerade and singlet-ungerade states are involved in the cascade excitation process of H2. The cascade from these states in the VUV provide diagnostic information on astrophysical sources for H2 emission.

A high-resolution (Δλ ~ 95 and 240 mA, FWHM) electron-impact-induced emission spectrum of molecular hydrogen has been obtained in the wavelength range 760-900 A. Analysis of the spectrum results in the assignment of some previously unobserved as well as observed but unassigned emission lines. Emissions observed in the 760-900 A range and a few selected regions above 900 A at room temperature (300 K) are tabulated, together with their spectral assignments. The tabulation can be considered as a supplement to the H2 atlas of Roncin & Launay and can be used for the analysis of EUV emission spectra observed in the interstellar medium and planetary atmospheres where H2 is the major constituent. Emission, predissociation, and autoionization yields for some transitions below 800 A are discussed. A Δv ≠ 0 perturbation between the Jj = 2 levels for vj = 5 of the D' 1Π state and vj = 2 of the 10pπ 1Π state is inferred.